A perfect-bound binding method, in which a body is composed of a layered product formed by jogging, stacking, and bundling a predetermined number of sheets, and then applying an adhesive on a rear face of the body, and if needed, in the vicinity of the face to attach a front cover to the body, is also adopted to bind notebooks or other kinds of booklets. As described above, in this binding method, an adhesive is applied on a face of the rear side (rear face) of the body to fix the back of the body. Hence, the booklet can be opened to the right and left from its joint, and the method is known to have advantages such as lower cost than saddle stitching binding, and more freedom in the number of sheets forming the body.
However, in this binding method, a rear end part of each sheet of the body is arranged and fixed perpendicular to the applied adhesive layer. For this reason, when an arbitrary page of the booklet is opened, normally, each sheet having its end fixed substantially perpendicular to the adhesive layer bulges upward particularly in the vicinity of the joint part, and therefore the sheets of the opened right and left pages do not open substantially 180 degrees. Thus, there has been a problem of insufficient spreading.
Additionally, in recent years, in binding of a perfect-bound booklet, a hot-melt type adhesive has been often used to attach the front cover to the rear face of the body. When the adhesive is applied, the low-viscosity adhesive melted by heating flows into the joint part between the body and the flyleaf (front cover), where the front cover and the rear end part of the first sheet in contact with the front cover are bonded. This causes a problem that the first sheet of the body cannot be opened favorably. Moreover, to improve adhesion of the sheets of the body and prevent them from dropping from the booklet, a kerf, a slit or the like substantially parallel to the thickness direction of the body is provided in the rear part of the body, and the sheets are bonded with an adhesive (notched or burst binding). Providing the kerf, slit or the like is effective in improving the adhesiveness of the sheets of the body to the inner face of the back part of the front cover. However, there has also been a problem that the adhesive infiltrates the inside of the body through the kerf to bond rear areas of the joint of adjacent sheets, whereby the pages cannot be spread 180 degrees.
As a result of extensive studies to solve the above problems, the applicant has developed a perfect-bound booklet (see Patent Literature 1) that can be spread favorably with right and left pages opened substantially 180 degrees without the vicinity of the joint part bulging upward when an arbitrary page of the body is opened, and the applicant has placed the product on the market as a flat notebook (registered trademark). The binding method described in Patent Literature 1 is aimed to bind a booklet in which an arbitrary folded signature in the body does not come off and drop from the body, and in a quality check after binding, the bonding strength of the rear glue has been checked thoroughly to see whether a signature does not come off and drop from the body.
However, recently, there have been many user requests for a perfect-bound booklet having contradictory functions of preventing a signature of the body from coming off and dropping in a normal state, but allowing a signature or a leaf to be cut off from the bottom of the joint when necessary, like a block memo pad.
Note that in the specification, “edge” of a booklet refers to a cut end (front edge) parallel to a spine of the booklet. In addition, “bundle” in “one-book bundle” or “multi-book bundle” collectively refers to a body formed of a predetermined number of sheets, a front cover, and a back cover.